1. Field of the Invention
This invention relates to a light-emitting device including a light-emitting layer made of a compound semiconductor. More specifically, the light-emitting device is for increasing a light extraction efficiency while a variation of an emission intensity and a wavelength depending on a radiation angle are controlled. The light-emitting device according to the present invention can be used for a device such as a lighting appliance, a backlight for a liquid crystal display (LCD), various types of indicator, a display panel, and the like.
2. Description of the Related Art
In recent years, as applications of a light-emitting diode (LED) increase, a demand for an increased optical output also has increased.
Concerning the demand, a way that a concavity and convexity are formed on a main surface of the light-emitting diode in a two-dimensional periodic structure, and the light extraction efficiency from the concave-convex surface is increased by use of a diffraction effect thereof, is proposed (referring to, for example, Japanese Patent Publication JP-A-2006-49855, (hereinafter, “patent document 1”)). In this way, the two-dimensional periodic structure (i.e., a photonic crystal) is formed on the main surface. A transmittance of the photonic crystal greatly depends on an incident wavelength and direction of light, and polarization (referring to, for example, Inoue Kuon, “Control of Optical Field by Using a Photonic Crystal”, Surface Science (The Surface Science Society of Japan), 2001, Vol. 22, No. 11 (hereinafter, “non-patent document 1”)).
As an example taking advantage of the above properties of the photonic crystal, a two-dimensional photonic crystal surface emitting laser is known, in which a polarization direction is controlled by making the photonic crystal asymmetric (e.g., referring to JP-B-3561244 (hereinafter, “patent document 2”)).
The structure according to patent document 1 is superior in increasing light extraction efficiency. However, when the structure is applied to a light-emitting diode, some problems occur.
Although the photonic crystal is extremely effective for being applied to a laser, there are some circumstances different from the photonic crystal being applied to a light-emitting diode. Firstly, the light-emitting diode has a light-emitting wavelength broader than that of a laser. Secondly, the light-emitting diode has a light radiation angle much wider than that of a laser. Therefore, since a transmittance of the photonic crystal depends on the incident wavelength and a direction of light as mentioned in the nonpatent document, the following problems occur.
Firstly, a light intensity varies depending on a radiation angle of the light-emitting diode. Secondly, a light-emitting spectrum varies depending on a radiation angle of the light-emitting diode. Specifically, a luminance and a color are viewed much differently depending on an angle looking into the light-emitting diode. The phenomenon can cause color or illumination shadings when the light-emitting diode is applied to an illumination or a backlight of an LCD.